feat(subagent): comprehensive sub-agent system with persistence

[lalyeah]

Summary

Implement a full sub-agent system that allows agents to spawn concurrent child agents with automatic result delivery, lifecycle management, and crash-safe persistence.

Core System (spoon_bot/subagent/)

• SubagentManager: Orchestration engine — spawn, cancel (cascade), steer, info, status
• SubagentRegistry: Thread-safe lifecycle tracking with state machine and JSON persistence
• SubagentTool: LLM-facing tool with spawn/status/cancel/kill/steer/info actions
• Models: SubagentConfig, SubagentRecord, SubagentResult, TokenUsage, SubagentState

Key Features

• Push-based wake continuation: Sub-agent results auto-injected into parent session via MessageBus
• Cascade kill: Cancelling a sub-agent recursively cancels all descendants (deepest-first)
• Steer: Redirect a running sub-agent to a new task mid-execution with rate limiting
• Pending descendants tracking: Prevents false "done" signals when children still running
• Token usage tracking per sub-agent run
• Extended thinking support (thinking_level config)
• Per-task run timeout via asyncio.wait_for
• Announce retry with exponential backoff (3 retries)
• Lifecycle events (SubagentEvent) emitted on the message bus
• Max spawn depth raised to 8

Persistence (persistence.py)

• Single JSON file: {workspace}/subagents/runs.json (schema version 1)
• Atomic writes (write-to-.tmp + replace()) for crash safety
• Startup restoration: Orphaned PENDING/RUNNING records → FAILED with diagnostic message
• Background sweeper: Archives terminal records after configurable interval (default 60min)
• Corrupt file quarantine: Renamed to .corrupt.json, graceful fallback to empty registry

Channel Integrations

• Telegram: /subagents command — list/spawn/cancel/kill/steer/info/help
  • spawn supports --model and --thinking flags
  • Enhanced list with active/recent separation, model names, pending descendants
• Discord: Lifecycle event listener logging spawn/complete/fail/cancel events

Configuration

New fields in SubagentLimitsConfig:

• persist_runs (default: true), persist_file, archive_after_minutes, sweeper_interval_seconds
• thinking_level, timeout_seconds in SubagentConfig

Test Plan

• All new models validate correctly (SubagentConfig, TokenUsage, SubagentRecord new fields)
• Registry count_pending_descendants() BFS works with parent-child hierarchies
• SubagentRunsFile save/load roundtrip preserves all fields
• Corrupt file recovery (quarantine to .corrupt.json, return empty dict)
• restore_subagent_runs() marks orphaned PENDING/RUNNING as FAILED
• Config max_depth=8 accepted
• Full test suite: 515 passed, 20 failed (all pre-existing), no regressions
• Manual: Telegram /subagents spawn/list/cancel/steer/info commands
• Manual: Restart gateway, verify runs.json restored and orphans reconciled